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Longitudinal Responses of the Equatorial/Low-Latitude
Ionosphere Over the Oceanic Regions to Geomagnetic
Storms of May and September 2017
A. O. Akala, E. O. Oyeyemi, P. O. Amaechi, S. M. Radicella, B. Nava,
Christine Amory-Mazaudier
To cite this version:
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1029/2020JA027963
Akala Andrew Oke-Ovie (Orcid ID: 0000-0002-9754-9360)
Longitudinal responses of the equatorial/low latitude ionosphere over the oceanic regions to geomagnetic storms of May and September, 2017
A. O. Akala1, 2, 3, 5, E. O. Oyeyemi1, P. O. Amaechi1, 4, S. M. Radicella5, B. Nava5, C. Amory-Mazaudier5, 6
1Department of Physics, University of Lagos, Akoka, Yaba, Lagos, Nigeria 2Distance Learning Institute, University of Lagos, Akoka, Yaba, Lagos, Nigeria
3Maritime Institute, University of Lagos, Akoka, Yaba, Lagos, Nigeria 4Department of Physical Sciences, Chrisland University, Abeokuta, Nigeria 5The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy
6 LPP, CNRS/Ecole Polytechnique/Sorbonne Université/Université Paris-Sud/Observatoire de
Paris, 4 Place Jussieu, 75252 Paris, France
Abstract
Ocean longitudes, particularly during the main phase of May, 2017 storm. Irregularities were generally inhibited by the September, 2017 storm.
Key words: Geomagnetic storms, ionospheric irregularities, total electron content, storm-time electric fields
1. Introduction
The occurrences of geomagnetic storms cause harmful disturbances in the geo-space environment [Chapman and Ferraro, 1931]. Geomagnetic storms are caused by solar features, such as; (i) the coronal mass ejections (CMEs) and, (ii) high speed streams (HSS) from the Sun’s coronal holes [Burlaga and Lepping, 1977, Gonzalez et al., 1994, Gopalswamy et al., 2009, Tsurutani et al., 2011]. CME has two geo-effective components; sheath fields [Guo et al., 2011] and magnetic cloud fields [Burlaga et al., 1981, Lepping et al., 2015]. Another driver of geomagnetic storms is the Co-rotating Interaction Region (CIR) and they (CIRs) are manifestations of HSS [Gosling, 1993, Gosling and
Pizzo, 1999].
Geomagnetic storms occurrences can significantly modify the ionosphere with resultant overbearing negative effects on space- and ground-based systems [NRC, 2008, Akala et al., 2013a, b]. One of the ionospheric parameters which are easily modified by the occurrences of geomagnetic storms is the Total Electron Content (TEC). TEC is the number of electrons in a column of 1 m2 cross-sectional
area between the Global Navigation Satellite System (GNSS) satellite and the receiver
[Hofmann-Wellenhof et al., 1994]. Variations in TEC can cause range errors for navigation signals in the order of
1 TECU to 0.163 m range error at GNSS L1 frequency (1.5754 GHz)[Klobuchar, 1996, Akala et al., 2013a, b]. It has been reported that sharp and rapid variations in TEC are sine qua non to occurrences of ionospheric plasma-density irregularities [Valladares et al., 1996, Akala et al., 2011]. Ionospheric irregularities cause scintillations of radio waves [Kintner et al., 2007], and severe scintillations on the other hand can cause loss of signal and cycle slips to trans-ionospheric radio systems [Akala et al., 2012, 2014, 2015, 2016, 2017].
During geomagnetic storms, equatorial/low latitude ionosphere is usually affected through the effect of PPEF [Vasyliunas 1970, 1972] and Disturbance Dynamo Electric Field (DDEF) [Blanc and
Richmond, 1980]. PPEFs are eastward during the day, and westward at night [Senior and Blanc,
1984]. Its magnetic signature, known as Disturbance Polar number 2 (DP2) [Nishida, 1968] is caused by electric current systems related to magnetospheric convection electric field [Nava et al., 2016,
Amory-Mazaudier et al., 2017]. Depending on its configuration, either eastward or westward, PPEF
Rayleigh Taylor Instability (RTI) [Abdu, 2012] is affected by the presence of PPEF. DDEF results from the dynamo action of storm time winds generated by Joule heating [Blanc and Richmond, 1980] and manifests as a negative excursion in the daily horizontal component of the Earth’s magnetic field. Its magnetic signature is referred to as the ionospheric disturbance dynamo (Ddyn) [Le Huy and
Amory-Mazaudier, 2005].
Fejer et al. [2008] reported the seasonal and longitudinal dependence of equatorial disturbance
vertical plasma drifts and concluded that opposite effects do exist between the magnitudes of PPEF and DDEF drifts across longitudes. As Fejer and Scherlies [1997] pointed out, prompt penetration drifts are mostly upward during the day and downward at night. The evening downward drifts have largest values in the eastern hemisphere, while the nighttime upward drifts maximize in the far Asia zone [Fejer et al., 2008, Fejer, 2011]. Another longitudinal study by Kuai et al. [2016] revealed a difference in the ionospheric response in the Asian-Australian and the American sectors due to the effects of PPEF and DDEF during the storm of March, 2015. In a similar effort, Patra et al. [2016] linked the occurrence of strong irregularities during the storm of March, 2015 to an eastward PPEF which caused a strong uplift of the F-layer over the Indian longitude, while Amaechi et al. [2018a, b] attributed the inhibited ionospheric irregularities over the African longitude to DDEF effects during the storm of March, 2015. These observations imply that PPEF and DDEF exhibit remarkable local time dependence which plays an important role in the longitudinal difference in the responses of the low latitude ionosphere to geomagnetic storms [Kuai et al., 2016].
From the standpoint of ground-based measurements, one cardinal hindrance to our global understanding of the storm-time electrodynamics of the equatorial/low latitude ionosphere is the paucity of ionospheric studies over the oceanic regions, which is largely due to the challenges of installing ionospheric sensors on the surface of the oceans. Paradoxically, oceans cover about seventy percent of the Earth’s surface. Although, over the past decades, satellite data have been quantitatively used to resolved the above concern [Burke et al., 2004, Gentile et al., 2006, 2011, Hei et al., 2005, Kil
et al., 2009a, b, Huang et al., 2014, Huang and Hairston, 2015, Carter et al., 2016, Akala et al., 2017, Astafyeva et al., 2018]. However, satellites measurements could have inherent challenges, such as
data adulteration by propagating medium interference, satellite altitude variations, instruments’ calibration error due to age, data archival and retrieval errors, amongst others. For data integrity control, validation of satellite data with corresponding ground-based data remains sacrosanct.
Akala et al. [2019] using ground-based GNSS receivers data, reported that coastal environments are
Consequently, the uneven heating of the atmosphere over the ocean leads to increase in atmospheric convections which create non-migrating tides that influence neutral wind circulations in the upper atmosphere [Fuller-Rowell et al., 1994], and invariably, influence the electrodynamics of the ionosphere [Hagan et al., 2001, Lau and Yang, 2015]. Furthermore, previous studies on tsunamis have also provided evidences of the existence of connections between ocean surface gravity waves and acoustic gravity waves in the ionosphere [e.g., Hickey et al., 2009, 2010, Makela et al., 2011,
Komjathy et al., 2012, Occhipinti et al., 2013]. However, these studies only investigated ionospheric
gravity waves after tsunamis had occurred, consequently, prediction of tsunamis prior to their occurrences, from the perspective of ionospheric dynamics remains an unresolved issue. Accordingly, detailed understanding of the physical processing involved in the ocean-ionosphere coupling is required for necessary parameterization and development of instrumentation for simultaneous monitoring of ocean surface gravity waves and ionospheric acoustic gravity waves on a global scale. From a theoretical perspective, Godin et al. [2015] connected surface gravity waves in the oceans with ionospheric acoustic gravity waves and their study was experimentally validated by Zabotin et al. [2016]. However, Zabotin’s et al. [2016] data analysis only covered a specific region in the North Atlantic. These authors recommended global validation of their results. To this end, this study investigates the longitudinal variations of storm-time electric field, characterized alongside with TEC and ionospheric irregularities over the global oceanic regions during 28th May and 8th September, 2017 storms.
2. Data and method of analysis
Two intense (-200 nT≤ Dst ≤ -100 nT) geomagnetic storms that occurred on the 28th of May, 2017 and 8th of September, 2017, respectively were studied. Interplanetary Magnetic Field (IMF) and solar wind plasma data were measured by the Advanced Composition Explorer (ACE) satellite. ACE data were obtained at https//omniweb.gsfc.nasa.gov/hw/hmtl. ACE satellite is located close to the L1 Lagrangian point (a distance of about 240 RE from the Earth). This large distance causes
which are often delayed before gaining momentum to increase at later times [Gosling, 1993, Gosling
and Pizzo, 1999].
The Auroral Electrojet (AE), Polar Cap North (PCN) and Polar Cap South (PCS) indices were obtained from the World Data Center (WDC) for Geomagnetism, Kyoto (htpp://wdc.kugi.kyoto-u.ac.jp). We used ground-based magnetometers’ data within the equatorial/low latitude strip around the oceanic regions to monitor changes in ionospheric currents during the storms. The magnetometers are owned by the Institut de Physique du Globe de Paris (IPGP) and data are made available via INTERMAGNET. The geographic locations of the GNSS and magnetometer stations are shown in Figure 1. SYM-H index data were used to compute ionospheric electric current disturbance (Diono). SYM-H and ASYMH data were obtained from isgi.unistra.fr. SYM-H profile changes rapidly the Earth’s magnetic field (H) and this also manifested clearly on the PC and AE profiles.
The horizontal component of the Earth’s magnetic field (H) was computed from the north component (X) and the south component (Y) of the field as H = √X2+ Y2. The mean quiet horizontal
component field was then estimated during five magnetic quiet days using equation (1).
∆H̅̅̅̅ =1
n∑ (Hquiet)i n
i=1 (1)
where n = 5 is the number of magnetic quiet days in a month.
We used the International Quiet Days (IQDs), selected from the German Research Centre for Geosciences website (ftp://ftp.gfz-potsdam.de) as the magnetic quiet days. The day-by-day variability was accounted for by taking the standard deviation of Hquiet during the magnetic quiet days. The
ionospheric electric current disturbance (Diono) which is the magnetic effect of the disturbed ionospheric electric field current systems were estimated [Shimeis et al., 2012, Fathy et al., 2014,
Azzouzi et al., 2015, Nava et al., 2016, Amory-Mazaudier et al., 2017].
Diono = ∆H – SYMH. Cosλ − Sq (2)
where ∆H in equation (2) is the observed component of the magnetic field, SYMH is an estimation of the ring current, λ is the geomagnetic latitude and ∆H̅̅̅̅ is the mean monthly quiet ∆H component of the Earth’s magnetic field. To first approximation, over the middle and low latitudes, Diono is the sum of DP2 and Ddyn [Le Huy and Amory-Mazaudier, 2005].
Diono = DP2 + Ddyn (3)
Ddyn from Diono, while high-pass filter with a maximum pass period of about 4 hours was used to extract DP2 from Diono.
We extracted Slant TEC (STEC) from GNSS observables data that were obtained from the University NAVSTAR Consortium (UNAVCO) website (htpp://www.unavco.org/data/data.html). The data were in Receiver Independent Exchange (RINEX) format and they were processed by Gopi TEC processing software [Seemala, 2010]. The carrier phase was leveled with pseudorange measurements. This further entailed detecting and correcting cycle slips [Blewitt, 1990] and calibrating STEC using satellite and estimated receiver biases [Ciraolo et al., 2007]. The calibrated STEC was converted to VTEC using mapping function at ionospheric pierce point of 350 km [Ma and Maruyama, 2003,
Mannucci et al., 1993]. Multipath effects were eliminated by adopting an elevation cut-off of 30o.
VTEC is hereafter referred to as TEC. Ground-based TEC measurements were validated with SWARM A TEC measurements at ftp://swarm-diss.eo.esa.int/. SWARM is European Space Agency’s constellation mission for Earth observation. The mission consists of three identical satellites (A, B, and C). SWARM A and C form the lower pair of satellites flying side-by-side (1.4° longitudinal separation) at 470 km altitude, while SWARM B flies at 520 km altitude.
We use rate of change of TEC index (ROTI) as proxy for ionospheric irregularities [Pi et al., 1997,
Oladipo and Schüler, 2013, Amaechi et al., 2018b, Zaourar et al., 2017, Yizengaw et al., 2018].
ROTI = √< 𝑅𝑂𝑇2 > −< 𝑅𝑂𝑇 >2 (4)
where ROT in equation (4) is the rate of change of TEC in TECU per minute.
An irregularities threshold of 0.5 TECU/min was set for all visible satellite at cut off elevation of 300.
Finally, we used JASON-2 satellite sea level anomaly data
(ftp://ftpcoastwatch.noaa.gov/pub/socd/lsa/johnk/coastwatch/j2) and optimum interpolation sea surface temperature data (https://www.ncdc.noaa.gov/oisst) to characterize the global ocean environments during the time intervals of the occurrences of the two storms. JASON-2 sea level anomaly data were not available during the 28th May storm. JASON-2 is a joint U.S./European Ocean Surface Tomography Mission flying at 1,336 km altitude.
3. Results
3.1 28th May, 2017 GS event
Figure 2(a–e) shows variations of solar wind speed, Vx (km/s), IMF Bz (nT), PCN and PCS indices
(mV/m), SYM-H (nT) and AE index (nT) from 23rd May–2nd June, 2017. The dash-lines on the figure represent forward shock. At about 1536 UT on the 27th of May, there was a forward shock that marked the arrival of a CME. Vx increased sharply to 367 m/s, while SYM-H gradually decreased,
attained peak values of about 9.0 mV/m, while AE reached peak values of about 1700 nT. SYM-H gradually increased on the 29th of May to about -5 nT, it thereafter decreased to -39 nT at 1900 UT. This period corresponded to the second southward turning of IMF Bz with a minimum value of -13.11 nT. There were also noticeable features on PCN, PCS and AE indices at 1230 UT on the 29th of May. The storm finally recovered on the 31st of May.
The zoomed plots of PCN, PCS, AE indices, ASYM-H and variations of Diono over Guam, Mbour and Kourou from 27–29th May, 2017 are shown in Figure 3(a–d). On the 27th of May, when the forward shock arrived at the Earth’s magnetosphere, an increase in Diono occurred only at Kourou. Oscillations in Diono with minima values of -10.17 nT, -28.86 nT and -46.29 nT for Guam, Mbour and Kourou, respectively within 2000–0000 UT. On the day of the storm’s main phase (28th of May), Diono reached three minima values: -60.19 nT, -51.79 nT and -46.66 nT within 0400–1020 UT (1400–2020 LT) at Guam and peaked to the positive at Mbour and Kourou. Later, minima values of -37.96 nT and -12.99 nT were recorded at about 1332 UT at Mbour (1332 LT) and Kourou (1032 LT). On the 29th of May, Diono exhibited some fluctuations with minima values at about 1200–1800 UT at the three stations (2200–0400 LT at Guam; 1200–1800 LT at Mbour and 0900–1500 LT at Kourou). Small variations in the magnetospheric pressure were very evident in PC, AE and ASYM-H measurements. A typical case was the obvious rise in amplitudes of the PC, AE and ASYM-H during daytime on the 29th May, 2017. Figure 4(a–d) shows the DP2 and Ddyn plots for Kourou, Mbour and Guam for 27–29th May, 2017. DP2 fluctuated during the main phase of the storm with short periods of less than an hour. The IMF Bz sharply turned southward around 2200 UT on the 27th of May, 2017, heralding the onset of the storm’s main phase. DP2 signatures were negative at 2200 UT, while Ddyn signatures were characterized with long periods of almost 24 hours. Consequently, Ddyn signatures play no major role during the main phase of the storm. There were also corresponding rise in auroral and polar cap indices at the onset of the main phase of the storm.
to 1330 LT and 1100 LT at Port Blair and Seychelles, respectively. Over the Atlantic Ocean longitude, TEC responded positively to the storm at Ascension Island and negatively at Dakar. At these two stations, UT = LT. The peak in TEC occurred at 1415 LT at Ascension Island, while a slight reduction in TEC was observed at Dakar. At the Pacific Ocean American longitude, TEC responded positively to the storm. The time of occurrence of the storm’s main phase corresponds to local sunset hours. A slight peak of TEC was observed over Galapagos at 2214 UT (1614 LT) but very pronounced at Tahiti, Mauna Kea and Cook Island. Figure 6(a–c) shows the global daily variations of SWARM A TEC on the 24th, 28th of May and 1st of June, 2017. 24th of May and 1st of June were quiet days with clear quiet TEC conditions. However, on the day of the storm’s main phase, 28th of May, TEC enhancements were observed. TEC enhancements were prevalent over the Pacific Ocean American and Pacific Ocean Asian longitudes and least over the Atlantic Ocean longitude. Figure 7(a–j) shows the behaviour of ionospheric irregularities from 23rd May–2nd June, 2017. Irregularities were not obvious over all the stations from 23–27th of May. On the 28th May, irregularities occurred over the Pacific Asian and American sectors. From 29th May–2nd of June, irregularities were again not conspicuous over all the stations. In terms of intensity, they were weaker over Guam and Galapagos and stronger over Tahiti, Mauna Kea, Cook Island and Solomon Island on the day of the storm’s main phase (28th May). Figure 8(a–c) shows the sea surface temperature during the May 2017 storm and for two days outside the storm. Sea surface temperature were higher over the Pacific Ocean Asian longitude, and lower over the Pacific Ocean American longitude, particularly, region very close to the South American coastlines.
3.2 8th September, 2017 GS event
Figure 9(a–e) shows variations of Vx (km/s), IMF Bz (nT), PCN and PCS indices (mV/m), SYM-H
(nT) and AE index (nT) from 3–13th September, 2017. The first CME released from the solar environment on the 4th of September at 1900 UT arrived at the Earth magnetosphere on the 7th of September in the morning [Tassev et al., 2017] as seen in the SSC. During this time, Vx and IMF Bz
increased abruptly. IMF Bz thereafter, turned southward with a value of -10.56 nT at about 0551 UT. The second CME arrived at the Earth at about 2330 UT on the 7th of September (vertical dash-lines). This storm is a dual-peak storm [Gonzalez et al., 1990]. SYM-H decreased to reach a minimum value of -132 nT at 0100 UT on the 8th of September, signifying the first minimum peak of the storm. From the time of the arrival of the second forward shock to the time of minimum decrease in SYM-H, Vx
On the 12th of September, Vx increased to 625.5 km/s and IMF Bz turned southward for the fourth
time with a minimum value of -11.79 nT. Around 2300 UT, PCN and PCN indices attained peak value of about 9.54 mV/m. AE recorded peak values of over 1000 nT. SYM-H reached a value of -53 nT at 0010 UT on the 13th of September.
The zoomed plots of PCN, PCS, AE indices, ASYM-H and variations of Diono over Guam, Mbour and Kourou from 3–13th of September, 2017 are shown in Figure 10(a–d). An increase in Diono was observed at Mbour (1200 LT) and Kourou (0900 LT) at about 1200 UT on the 6th of September and 1445 UT (1445 LT at Mbour and 1145 LT at Kourou) on the 7th of September. This was not noticeable over Guam because of local time effect (early morning hours). In the early hours of 8th September at about 0030 UT (1030 LT at Guam, 0030 LT at Mbour and 2130 LT at Kourou), a sharp increase in Diono of magnitude 28.31 nT, 103.9 nT and 38.87nT occurred at the three magnetometer stations, respectively. This was followed by a decrease to a minimum of -47.35 nT at about 0200 UT (1200 LT) over Guam only. From noon to post-noon hours, Diono fluctuated with minimum values of -43.32 nT at 1530 UT (0130 LT) at Guam; -112.2 nT at 1430 UT (1430 LT) at Mbour; and -70.42 nT at 1513 UT (1213 LT) at Kourou. On the 10th of September, the three magnetometer stations experienced some enhancements in Diono at about 1610 UT (0210 LT at Guam, 1610 LT at Mbour and 1310 LT at Kourou). Figure 11(a–d) shows the DP2 and Ddyn plots for Kourou, Mbour and Guam for 7–9th September, 2017. DP2 fluctuated during the main phase of the storm. The IMF Bz sharply turned southward around 2200 UT on the 7th of September, 2017, heralding the onset of the storm’s main phase with short periods of less than an hour. DP2 signatures were negative around 2100 UT, while Ddyn signatures were characterized with long periods of almost 24 hours. Consequently, Ddyn signatures play no major role during the main phase of the storm. There were also corresponding rise in auroral and polar cap indices at the onset of the main phase of the storm.
morning up to about 0600 UT (1400 LT). Over the Indian Ocean longitude, the influences of the storm on TEC were not noticeable at Seychelles on the day of the storm’s main phase. There were data gaps on the 8th of September at Diego Garcia. The time of occurrence of the storm’s main phase corresponded to post sunset hours at this longitude. Over the Atlantic Ocean longitude, TEC responded positively to the storm. The time of occurrence of the storm’s main phase corresponded to daily active solar period: 1400 and 1500 LT at Fortaleza and Kourou, respectively and 1800 LT at Dakar and Ascension Island. Over the Pacific Ocean American longitude, TEC responded positively to the storm. The time of occurrence of the storm’s main phase corresponded to local sunset hours. Figure 13(a–c) shows the global daily variations of SWARM A TEC for 4th, 8th and 12th of September, 2017. Surprisingly, despite the fact that 4th of September was a magnetic quiet day, TEC enhancements were generally observed over the oceanic longitudes. These enhancements were most prevalent over the Pacific Ocean Asian and Pacific Ocean American longitudes, followed by the Indian Ocean longitude and least over the Atlantic Ocean longitude. 12th of September was a quiet day with obvious quiet TEC condition. On the day of the storm’s main phase, 8th of September, TEC enhancements were prevalent over the Pacific Ocean Asian, Pacific Ocean American and the Atlantic Ocean longitudes and least over the Indian Ocean longitude.
4. Discussion
The two storms were caused by CME transients, i.e., manifestations of magnetic fields reconfigurations driven by magnetic reconnections [Dungey, 1961] at the Sun’s coronal surface. The arrival of CMEs at the magnetosphere created the observed fast forward shocks that led to the SSCs for both storms (Figures 2 and 9). CME transient has two components, namely, magnetic cloud (a magnetic flux rope) [Burlaga et al., 1981, Lepping et al., 1990, 2015] and turbulent magnetic field regions (sheath regions) [Guo et al., 2011]. These regions are usually sandwiched in-between shocks and the leading edges of CMEs. Specifically, the storm of 28th of May, 2017 was driven by sheath and magnetic cloud fields, while the storm of 8th of September, 2017 was driven by sheath fields.
The transit of solar eruptions from solar surface to the Earth’s magnetosphere usually leads to transfer of energy into the magnetosphere [Sckopke, 1966]. During the occurrences of the two storms, there were enhancements in auroral currents (Figures 3 and 10). The influence of these high auroral currents manifested clearly in the enhancements of the observed auroral/high latitude TEC measurements by SWARM A satellite (Figures 6 and 13), particularly, over the Pacific Ocean American and Pacific Ocean Asian longitudes. Electric currents at auroral latitudes get enhanced by precipitating particles, field aligned currents and convection of magnetospheric electric field via Joule heating and momentum transfer by the Ampere force. Joule heating leads to transfer of energy to the neutrals, while Ampere force moves neutral wind by momentum transfer. Both processes drive thermospheric wind [Richmond and Roble, 1979; Sharma et al., 2011], which could extend from the high- to middle- and low latitudes [Mazaudier et al., 1985, Nava et al., 2016]. These winds drag ionization to altitudes where recombination rates are significantly reduced, thereby causing enhancements in TEC, electron plasma density and other ionospheric parameters. These winds are responsible for global changes in thermospheric composition [Fuller-Rowell et al., 1994] and they are longitudinal dependent [Jones, 1971, Prolss, 1995].
corresponds to around 1200–1300 LT (daytime) over the Pacific Ocean American longitude. SYM-H gradually decreased, reaching a minimum value of -145 nT at 0800 UT on the 28th of May. 0800 UT corresponds to 2100–2200 LT (post-sunset) over the Pacific Ocean American longitude, 0500–0800 LT (early morning) over the Atlantic Ocean longitude, 1100–1300 LT (daytime) over the Indian Ocean longitude and 0400–0500 LT (early morning) over the Pacific Ocean Asian longitude. During storms, high latitudes electric fields, when sufficiently large do penetrate promptly into the low latitude ionosphere [Nishida, 1968]. This electric field, termed PPEF [Vasyliunas, 1970; Senior and
Blanc, 1984] is eastward and westward in the dayside and night-side, respectively [Hanson and Moffett, 1966, Abdu, 2012, Abdu et al., 1997]. Short-term oscillations of Diono with a period around
30 minutes to roughly lesser than 3 hours, during southward turning of IMF Bz were very evident in DP2. DP2 are signatures of PPEF, while Ddyn are signatures of DDEF [Nishida et al., 1966, Hairston
et al., 2016, Amory-Mazaudier et al., 2017].
In the Pacific American longitude, during daytime, Tahiti (-16.42o dip) and Cook Island (-21.61o dip)
recorded conspicuous increase in TEC, maximum TEC at 1730 LT and 1756 LT, respectively. Galapagos (9.46o dip) and Mauna Kea (20.53o dip) recorded comparatively lesser enhancements in
TEC, maximum TEC at about 1315 LT and 1315 LT, respectively (Figure 5). Over Solomon Island and Guam in the Pacific Ocean Asian longitude, enhancements in TEC were also observed. Over the Indian Ocean and Atlantic Ocean longitudes, being in the night-side during the storm onset, the storm occurrence did not cause significant influence in TEC over the two longitudes. The enhancements in TEC were due to the eastward PPEF that occurred in the daytime. During daytime storm’s onset, under southward turning of IMF Bz, eastward PPEF enhances low latitude ionospheric electrodynamics [Kikuchi et al., 1996]. The observed response of the ionosphere over the Pacific American longitude was an indication of a strong fountain effect that gave rise to increase in ionization over the crests (Tahiti, -16.42o dip and Cook Island, -21.20o dip). However, Mauna Mkea
(20.53o dip) experienced lower enhancement in TEC compared to TEC observations at Tahiti and
Cook Island. This was a clear indication of asymmetry in the redistribution of plasma over the northern and southern crests of the EIA during the disturbance [Huang et al., 2012, Immel and
Mannucci, 2013]. The hemispherical asymmetry in plasma distribution is attributable to the effects of
neutral winds [Zaourar et al., 2017]. Over the Atlantic African longitude, Dakar (5.88o dip) located at
the trough of the EIA experienced TEC reduction while Ascension Island (-16.09o dip) located toward
2014]. DDEF is opposite in configuration to PPEF and longer in period (in the order of days), as such reduced TEC during the recovery phase of storms.
The 8th September, 2017 daytime has a unique characteristic of two-step main phase. The arrival of the first shock was around the midnight of 6th of September. The second shock arrived almost at midnight of 7th of September but the IMF Bz turned sharply southward around 2200 UT on the 7th of September, 2017 to herald the onset/SSC of the daytime and caused SYM-H to rise suddenly in the positive direction (Figure 9). The convection current also increased correspondingly during the time of the storm onset (Figure 10). At the storm onset, Diono increased at Kourou, and decreased at Mbour and Guam but reversed in orientation during the main phase of the storm, positive over Mbour and Guam, and negative over Kourou. At the storm onset, DP2 signatures fluctuated into negative direction, while the Ddyn signatures were characterized with longer-hours periods at the three magnetometers stations (Figure 11). 2200 UT corresponds to 1900 LT (post-sunset hours) at Kourou, 2200 LT (post-sunset/nighttime) at Mbour, and 0800 LT (daytime) at Guam. This time corresponds to around 1100–1200 LT (daytime) over the Pacific Ocean American longitude. SYM-H gradually decreased to -132 nT at 0100 UT on the 8th of September, 2017. 0100 UT corresponds to 1400–1500 LT (daytime) over the Pacific Ocean American longitude, 2300–0100 LT (nighttime) over the Atlantic Ocean longitude, 0400–0600 LT (early morning) over the Indian Ocean longitude and 1100– 1200 LT (around midday) over the Pacific Ocean Asian longitude
TEC enhancements were observed over the Pacific Ocean Asian and Pacific Ocean American longitudes (Figure 12). These two longitudes were in the dayside during the occurrence of PPEF. Surprisingly, Tahiti (-16.42o) experienced comparatively lower enhancements in TEC over the Pacific
Ocean American longitude. This could be an indication that the crest of the EIA in the southern hemisphere of this sector had moved ahead of Tahiti (-16.42o) by super fountain effect. Perhaps, PPEF
longitudes. The increase in TEC was very obvious on the 4th of September (Figure 13). The enhancement in TEC observed on the 4–6th of September, despite the fact that these days were magnetically quiet days might be attributed to the solar flares that occurred during the period (www.spaceweather.com). As reported by Yasyukevich et al. [2018], these flares were accompanied by solar radio bursts, leading to increase in TEC. It is well known that X-rays and Extreme Ultra Violet (EUV) radiation have the capacity of increasing ionization in the ionosphere, especially on the dayside [Mitra, 1974, Tsurutani et al., 2009, Yan et al., 2018].
Generally, over the equatorial/low latitude region, post-sunset eastward Pre-Reversal Enhancement (PRE) of 𝐸 × 𝐵 drift supports the uplifting of F-layer altitude to create conducive environment where irregularities can be generated through the Rayleigh Taylor Instability (RTI) process [Abdu et al., 1995, Akala et al., 2014, 2016]. However, if the electric fields are westward during post-sunset hours in the region, the uplifting of 𝐸 × 𝐵 drifts is prevented, consequently, irregularities generation is inhibited. For the May, 2017 storm, irregularities were observed on the days before and after the day of the main phase of the storm. During the main phase of the storm, irregularities were triggered over stations located in the Pacific Ocean American longitude (Figure 7). The main phase of the storm occurred at a period that corresponded to post-sunset hours at this longitude, the hours of the day which supports PRE generations. The generated PREs might have strengthened 𝐸 × 𝐵 drift, thereby lifting the F-layer to altitude where the formation of irregularities was favourable [Sultan, 1996,
Amaechi et al., 2018a]. For the September, 2017 storm, irregularities were not conspicuous over all
the longitudes (Figure 14). The times of the storm’s onset and the storm main phase coincided with early morning, daytime and nighttime hours over all the longitudes. These hours do not support PRE generation. Consequently, irregularities were not observed over the longitudes. The irregularities inhibition on the 8th of September was driven by westward electric fields (DDEF). Over the Atlantic Ocean longitude, irregularities were present on the 7 and 10–11th of September; they were triggered by the eastward PPEF associated with the second turning of IMF Bz [Huang et al., 2017].
ENSO has two phases, namely, negative (El Nino) and positive (La Nina). El Nino involves anomalously warm sea surface temperatures, in the eastern and central Pacific Oceans which produces large changes in evaporation and latent heat transfer to the atmosphere [Zeng and Pyle, 2005].
The sea temperature anomaly causes the warmest sea surface temperature and convection that are normally found in the western Pacific Ocean at the expense of the eastern Pacific Ocean (Figure 8 and 15). These often lead to variations in surface pressure between the western and eastern Pacific Oceans. Low pressure is shifted from the western Pacific and Indian Oceans to the central and eastern Pacific Oceans. During El Nino, low pressure is anomalously low over the central and eastern Pacific Oceans, whereas the western Pacific Ocean experiences anomalously high pressure. The opposite phase of the EL Nino is La Nina with anomalously low pressure over the Indian Ocean and western Pacific Ocean and anomalously high over the central and eastern Pacific Ocean [Zeng and Pyle, 2005]. The dominance of intensity of TEC over the eastern and western Pacific Oceans manifested clearly in our data (Figures 6 and 13). Furthermore, intensities of TEC were generally higher over the oceans than over the landlocked regions. We recorded stronger irregularities over the western Pacific Ocean longitude during the time when the sea surface temperature was significantly reduced. Negative sea surface temperature anomaly increases the wind system in the entire atmosphere and non-migrating waves in the upper atmosphere. This could have in turn enhanced ionospheric dynamo electric field. It is important to point out that the data used for the current analysis only spanned for a period of eleven days; five days before the main phase of each storm, and five days after, in addition to the day of the main phase of the storm. For this reason, to strengthen the conclusion on the possible influences of the ocean on the ionosphere, future analyses of large volume of data over long period of time are recommended.
5. Conclusions
This work investigated the responses of the equatorial/low latitude ionosphere over the global oceanic regions to geomagnetic storms of May and September, 2017. We concluded that:
1. Both storms were caused by CME transients. A positive storm effect in TEC caused by eastward PPEF was observed over most of the longitudinal sectors on the day of the first storm (28th May, 2017). The largest and long duration enhancements were recorded over the Pacific Ocean American longitude. PPEF also intensified the southern crest of the anomaly over the Pacific Ocean American longitude. The responses of TEC over the Indian Ocean and Atlantic Ocean longitudes were mild. The longitudinal dependence of PPEF is premised on its local time of storm occurrence.
responses to the storm were generally low over the Atlantic Ocean and Indian Ocean longitudes because of the local time effects of PPEF. TEC enhancements observed on the 4–6th of September, geomagnetic quiet days were associated with the solar flares occurrences [Tassev et al., 2017, Lei et
al., 2018]. Furthermore, DDEFs generally manifested by weak Ddyn signatures after the main phases
of the two storms were responsible for positive ionospheric responses during the recovery phases of the storms.
3. The first storm triggered irregularities over the Pacific Ocean longitude during lager PC and AE currents within 0700–1200 UT (for instance, this time range corresponds to 1830–2330 LT at Tahiti and 1300–1800 LT at Galapagos) on the storm day. This time range is the period for PREs, a time conducive for irregularities generation. For the second storm, irregularities were inhibited by westward DDEF occurrence during daytime morning/afternoon. This westward electric field cancels the PRE eastward electric field; hence hindering the generation of irregularities. These results validated model results by Blank and Richmond [1980] and Fejer et al. [2008]. Furthermore, our results are in agreement with earlier results by Nayak et al. [2016a, b] and Amaechi et al. [2018a].
4. The intensity of sea surface temperature and pressure has dependence on ionospheric TEC and irregularities. Our results showed that intensities of TEC were higher over the oceans than over the landlocked regions. The coupling between the ocean and the atmosphere, especially over the Pacific Ocean Asian and American longitudes has a relation with Walker's circulation. Walker’s circulation modifies the convection motions of the atmosphere. Atmospheric convection generates non-migrating tides propagating at the altitudes of the ionosphere and modifying the atmospheric circulation and as a consequence, influencing the dynamo electric field and invariably, ionospheric irregularities generation.
Acknowledgments
Ddyn and DP2 signatures from Diono. The authors also thank the editor and the reviewers for their useful comments and suggestions which have significantly improved this paper.
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Figure 12: VTEC (red curve), monthly mean quiet TEC (back curve) and standard deviation use to represent the day-day variation in VTEC (cyan area) from 3rd–13th September, 2017 over: Pacific Ocean (PFO) Asian longitude:
(a) Solomon Island [solo]; (b) Namria General Santos [pgen], Indian Ocean (IDO): (c) Diego Garcia [dgar], (d) Republic of Seychelles [sey2]; Atlantic Ocean (ATO): (e) Ascension Island [ascg], (f) Dakar [dakr], (g)Fortaleza [brft], (h) Kourou [koug]; Pacific Ocean (PFO) American longitude: (i) Risiketea [gamb], (j) Tahiti [faa1],
